The production of block copolymers may be accomplished through the use of catalysts that undergo rapid and reversible chain transfer to a main-group metal in combination with two or more reactors in series that are operated under different process conditions. To enable this technology, it is useful to identify catalysts and process conditions under which rapid and reversible chain transfer to suitable chain transfer agents occurs. Such technology could be useful in making block copolymers.
A “block copolymer” is a polymer consisting of multiple sequences, or blocks, of the same monomer alternating in series with different monomer blocks. The blocks are covalently bound to each other such as AAAABABBABAAAA fashion (A and B are different types of monomers). Block copolymers are classified based on the number of blocks they contain and how the blocks are arranged. For example, block copolymers with two blocks are called diblocks; those with three blocks are triblocks; and those with more than three are called multiblocks. Classifications by arrangement include the linear, or end-to-end, arrangement and the star arrangement, in which one polymer is the base for multiple branches. One common example of block copolymer use is in the making of shoe soles and tire treads. These items are constructed out of SBS rubber, or polystyrene-butadiene-styrene), which is a hard rubber.
The importance of block copolymers can be seen in their wide array of properties. These properties are made possible due to the combination of different polymers in alternating sequence. An example of such property manipulation can be seen in poly(urethane) foams, which are used in bedding and upholstery. Poly(urethane), a multiblock copolymer, is characterized by high-temperature resilience and low-temperature flexibility.
Another important use of block copolymers is in industrial melt-adhesives. By combining polystyrene or polyolefins with polymers which exhibit rubber-like and adhesive properties, sturdy adhesives can be formed which are activated by heat. The structure of this copolymer utilizes polystyrene or polyolefin blocks on the outside and the rubber block on the inside. When heat is applied, the polystyrene or polyolefin parts melt and allow for limited liquid-like flow. The middle section causes adhesion and after the temperature drops, the strength of block copolymer is restored. This property, made possible by the combination of polyolefins with other polymers, makes this block copolymer an important adhesive.
WO 2007/130306 discloses a class of transition metal imidazoldiyl catalysts that are useful for making broad molecular weight distribution polyolefins, but has not been shown to be useful in making the blocky copolymers that are desired. Pyridyldiamide catalysts have been described in U.S. Pat. No. 7,973,116, U.S. 2011/0224391, U.S. 2011/0301310, and U.S. 2012/0071616. These pyridyldiamide catalysts are useful in making very narrow molecular weight distribution (less than 2.5) polyolefins. What is needed is a method of using this type of catalyst in producing narrow molecular weight blocky polyolefins.
Other references of interest include U.S. Pat. No. 7,858,706.